1. Field of the Invention
This invention relates generally to wireless communications, and more particularly to a method of recovering a network time base for radio data demodulation after an IDLE period of communication.
2. Description of the Prior Art
A mobile terminal is required to keep itself permanently synchronized on a network time to accurately demodulate data received through a radio link. Further, the accuracy of a base-station oscillator (e.g., 0.01 ppm) is better than the accuracy of a mobile oscillator (e.g., 0.3 ppm). This difference in oscillator accuracy causes the local time associated with the mobile terminal to drift away from the network time associated with the base-station. Periodic re-synchronization is therefore required to minimize the foregoing time drift consistent with maintaining demodulator performance associated with the mobile terminal. The re-synchronization operation is based on the demodulation of received data which includes a predetermined training sequence to compute time and frequency differences between the base-station and the mobile terminal.
During IDLE mode, no radio transmission occurs, thus removing any possibility of re-synchronization. Since the IDLE period duration is defined by a radio standard protocol, the accuracy of the mobile terminal time-base clock must be high enough to allow a global time drift in the time range imposed by the demodulator performances.
Present methods of re-synchronization require constant activation of a mobile terminal high-frequency local oscillator to maintain the synchronization of the mobile time with the network time during IDLE periods. Such methods lead to high current consumption during an IDLE mode since the mobile terminal high-frequency oscillator and its associated clocking elements and counting logic will be active. A significant disadvantage of these present methods relates to shortened battery life and therefore increased operating costs associated with mobile communication terminals.
In view of the foregoing, it would be desirable to have a method of re-synchronizing a mobile communication terminal on the network, e.g., global system for mobile communication (GSM) network, during inactive communication periods, i.e., IDLE mode.
The present invention is directed to a method of recovering a network base-station time base for radio data demodulation within a mobile terminal after an IDLE period of communication. The method gauges a low frequency local oscillator within the mobile terminal versus the network time of a radio-communication network. A local time-base within the mobile terminal stores the network time while using a local and highly stable high-frequency oscillator. The local high-frequency oscillator is shut down during the IDLE period. The local time-base is periodically updated during non-IDLE periods with a time delta correction resulting from a demodulation of received communication data. The equivalent elapsed network time during the IDLE period is computed from a mobile terminal counter using a local low-frequency counter that is clocked via the low frequency local oscillator. The local low frequency oscillator is gauged versus the local time-base with the sampling of the time-base value when starting and stopping the gauging. The gauging process does not require updating of the local time-base during the gauging period. The local time-base is then restored subsequent to an inactive period (IDLE) and having sufficient accuracy to reliably demodulate communication data. This gauging process eliminates the necessity for maintaining the high frequency local oscillator within the mobile terminal during the gauging process, thereby providing for interlacing of the gauging and IDLE periods. This gauging process further makes the accuracy of the restored local time-base independent of the duration of the inactive period (IDLE).
As used herein, the following words have the following meanings. The words xe2x80x9calgorithmic softwarexe2x80x9d mean an algorithmic program used to direct the processing of data by a computer or data processing device. The words xe2x80x9cdata processing devicexe2x80x9d as used herein refer to a CPU, DSP, microprocessor, micro-controller, or other like device and an interface system. The interface system provides access to the data processing device such that data could be entered and processed by the data processing device. The words xe2x80x9cdiscretexe2x80x9d data as used herein are interchangeable with xe2x80x9cdigitizedxe2x80x9d data and xe2x80x9cdigitizedxe2x80x9d data as used herein means data which are stored in the form of singularly isolated, discontinuous data or digits.
One embodiment of the present method of keeping a local time-base in a mobile terminal synchronized with a network time-base during a network IDLE transmission period comprises the steps of:
a) starting a low frequency gauging counter C to track local low frequency (LF) oscillator cycles associated with the mobile terminal;
b) sampling the network time-base near the end of a network data transmission period via a local high frequency (HF) oscillator associated with the mobile terminal, upon starting the gauging counter C, to determine a network time-base value TSLEEP;
c) starting an IDLE transmission period LF counter at time TSLEEP;
d) stopping the local HF oscillator upon starting the IDLE transmission period LF counter;
e) stopping the IDLE transmission period LF counter at time TWAKE-UP, where TWAKE-UP is dependent on the network time-base value TSLEEP, a count value LF stored by the IDLE transmission period LF counter during the IDLE transmission period, and a gauging factor Restimated associated with a most recently previous network IDLE transmission period for the local LF oscillator in HF oscillator period units according to the relationship defined by TWAKE-UP=TSLEEP+((Restimated)Previous*LF)+EIDLE;
f) starting the HF oscillator at time TWAKE-UP to implement demodulation of network time-base transmission signals and thereby recover the network time-base such that the mobile terminal can be synchronized with the network time-base;
g) stopping the low frequency gauging counter C upon recovery of the network time-base and determining an elapsed count value associated with the gauging counter C;
h) generating a new gauging factor (Restimated)New for the LF oscillator in HF oscillator period units associated with the mobile terminal wherein the gauging factor Restimated is dependent on the elapsed count value associated with the gauging counter C; and
i) repeating continuously steps a-h.
A structure suitable for implementing the present method comprises a mobile terminal system for keeping a local time-base synchronized with a network time-base during a network IDLE transmission period, the system comprising:
an IDLE period timer;
a data processing device;
an algorithmic software directing the data processor; and
a data storage unit, wherein discrete IDLE period data, discrete low frequency oscillator data, discrete high frequency oscillator data, discrete sampling error data, discrete algorithmic error data, discrete low frequency oscillator jitter data, discrete network time-base data, discrete rounding data, discrete low frequency oscillator aging data, and discrete low frequency oscillator drift data are stored and supplied to the data processing device such that the data processing device, directed by the algorithmic software, can automatically determine a time-base value TWAKE-UP, using algorithmically defined relationships among the discrete data and thereby cause the IDLE period timer to modulate the IDLE transmission period in response to the time-base value TWAKE-UP, such that the mobile terminal can function to be synchronized with the network time-base.
A feature of the present invention is associated with recovery of a network time-base necessary to accommodate communication data demodulation following an IDLE period via a local low frequency/low accuracy oscillator within a mobile terminal.
Another feature of-the present invention is associated with a synchronization method rendering a mobile terminal capable of maintaining synchronization during IDLE periods (no radio traffic) without using a local high frequency oscillator.
Still another feature of the present invention is associated with a method of operating a mobile terminal during IDLE periods such that the mobile terminal power consumption is substantially reduced.